1. Field of the Invention
The present invention relates to a method of recording optical information and an optical recording system for recording optical information onto an optical data recording medium, primarily an optical disc which has a function of recording and reproducing optical data at a high speed with high density by using a single laser beam and the like.
2. Description of the Prior Art
There have been known techniques of recording and reproducing optical data at a high speed with high density by using a laser beam, which have been put into practical use primarily as optical discs. The optical discs can be briefly classified into three broad categories, i.e., read-only (reproduction exclusive), write-once read-many (additional writing), and rewritable types. The read-only discs have been commercialized as compact discs for recording audio data, or as laser video discs for recording video data. The write-once read-many discs have been commercialized as document files, still image files, and the like. At present, research and development are being advanced, focused on the rewritable discs, which are increasingly commercialized as data files for personal computers and the like.
The rewritable optical disc incorporates a recording thin film having an optical characteristic that a reversible change between two or more states is induced when there is a change in irradiation conditions of laser beams or the like. The rewritable optical discs are typically classified into two types, i.e., magneto-optical type and phase-change type.
In the magneto-optical type, a ferromagnetic thin film is used as the recording thin film, so that a signal is recorded by changing the orientation of the magnetic domain.
In the phase-change type optical discs, a recording operation of optical signals is performed in such a manner that the recording film is changed in phase reversibly between an amorphous phase and a crystalline phase by changing the irradiation conditions of the laser beam thereby to record the optical data in the optical disc, while a playback operation of recorded data is performed by optically detecting differences in optical reflectance between the amorphous and crystalline phases of the recording film thereby to reproduce the recorded data. Accordingly, the phase-change type optical discs are capable of reproducing a recorded data signal as a change in reflectance of a laser beam, in the same manner as in the read-only type and the additional writing type discs. Besides, the phase-change type optical discs advantageously allow the optical recording system construction to be simplified by virtue of its capability of overwriting with a single laser beam by modulating the laser power between an erasing level and a recording level thereof.
As a method of recording optical data signals onto a rewritable optical disc already commercially available, there is employed in most cases a pulse position modulation method (hereinafter, abbreviated as "PPM"), in which the positions of individual recording marks each corresponds to a level "1" of the digital signals to be recorded. However, for further enhanced density, the pulse-width modulation (hereinafter, abbreviated as "PWM") is now under discussion, where the edge positions at the front and rear of a recording mark each corresponds to the level "1" of the digital signals. It is noted-here that the front edge corresponds to the laser beam irradiation starting portion of the recorded mark while the rear edge corresponds to the laser beam irradiation ending portion of the recorded mark.
In the PWM system, since the width of the recorded mark carries optical data, it is necessary that the recorded mark be free from distortion in width, in other words, the recorded mark should have a symmetrical shape between the leading edge and the trailing edge thereof. However, in a recording operation of recording an optical data signal onto an optical disc, in a laser beam application portion of the recording film, there is caused a higher temperature portion at the trailing edge where the laser beam irradiation is ended due to a heat accumulation effect and there is caused a lower temperature portion at the leading edge where the laser beam irradiation is started. That is, the achieved temperature of the recording film is relatively low at the front and gradually increases toward the rear due to the effects of preheating. Therefore, the recorded mark is made wider at the trailing edge than at the leading edge, so that there is a problem that the shape of the recorded mark is narrower at the leading edge and wider at the trailing edge, resulting in distorted in a shape of tear-like drop. This would cause a distortion of a playback signal waveform, such that in some cases a recorded signal could not be correctly reproduced.
Therefore, the inventors of the present invention proposed an overwriting recording method as disclosed in the U.S. Pat. No. 5,109,373 which is assigned to the same assignee as the present application. In this U.S. Patent, one recorded mark is formed by irradiation of a laser beam with a plural-short-pulse string for reducing the aforementioned distortion of a recorded mark. This method, however, has involved with a possible problem that, if a relative velocity of a laser beam spot differs with respect to a position of the disc surface, for example, between inner and outer circumferences of the disc surface when the optical disc is rotated with a constant speed, there may arise lack of laser power especially in the outer circumference region where the relative velocity of a laser beam spot is high, involving a difficulty in circuit design, to a newly developed problem.
In more detail, with reference to FIGS. 9(a) through 9(d), in the conventional methods, first described is the cause of the fact that the recording mark distorts into a tear-like shape when optical data is recorded onto an optical recording disc. FIG. 9(a) shows a waveform of an input digital signal to be recorded having pulse duration periods and pulse spacing periods. In the first conventional method, as shown in FIG. 9(b), the recording input signal is directly converted into a laser beam output modulation power by modulating the laser output power level in a range between an erasing power level Pb and a recording power level Pp, where the pulse duration periods of the input signal directly correspond to the recording power level Pp while the pulse spacing periods correspond to the erasing power level Pb.
In this case, as shown in FIG. 9(c), the achieved temperature of the recording film of the optical disc would be higher at the laser irradiation ending portion of the recorded mark than at the laser irradiation starting portion of the recorded mark due to the heat accumulation effect (i.e., preheating effect). The laser irradiation ending portion corresponds to the trailing edge of the recorded mark, while the laser irradiation starting portion corresponds to the leading edge of the recorded mark. As a result, the shape of the recorded mark would be wider at the trailing edge than at the leading edge to be distorted into a tear-like shape, as shown in FIG. 9(d). Since the heat accumulation effect will increase with lowered relative velocity between the optical disc and the laser beam spot (hereinafter, referred to as "linear velocity"), therefore the tear-like distortion also enlarges with lowered linear velocity. This tear-like distortion would cause a distortion in a playback signal waveform, and therefore it has been impossible in some cases to reproduce a recorded signal correctly.
In order to reduce such a tear-like distortion of recorded marks, the inventors of the present invention proposed an overwriting recording method in the U.S. Pat. No. 5,109,373 in which one recorded mark is formed by laser beam irradiation with a multiple-short-pulse string.
The second conventional overwriting method mentioned above is next explained with reference to FIGS. 10(a) through 10(d), where an input digital recording signal as shown in FIG. 10(a) is overwritten by a PWM method with conversion into a multiple-short-pulse string, thereafter modulating the laser output power level in a range between an erasing power level Pb (Low) and a recording power level Pp (High) as shown in FIG. 10(b) in accordance with the corrected input pulse string waveform. It is to be noted here that the short-pulse string consists of one leading pulse (head pulse) having a wider pulse width and a succeeding pulse string each having a narrower pulse width. The width of the leading pulse is made constant independently of the length of a recording mark, while the pulses in the succeeding pulse string are equal both in width and in interval to one another, where the number of the pulses contained in the succeeding pulse string is n-l to form the n-th shortest entry recording mark in length where n is a positive integer.
For example, the 8-14 modulation signal (hereinafter, abbreviated as "EFM signal"), which is adopted for compact discs, is formed of pulses of nine types in length from 3 T to 11 T (T denotes a clock period, hereinafter). When the EFM signal is used as an input signal to be recorded, the shortest 3 T pulse is converted into a short-pulse string consisting of only the leading head pulse of 3 T in length without any succeeding pulse, the second shortest 4 T pulse is converted into a short-pulse string consisting of one leading pulse and one succeeding pulse, the third shortest 5 T pulse is converted into a short-pulse string consisting of one leading pulse and two succeeding pulses, and so on, and thus the longest (ninth shortest) 11 T pulse is converted into a short-pulse string consisting of one leading pulse and eight succeeding pulses. By such regularly arranged conversion, a signal conversion circuit can be simplified in construction. In this case, the achieved temperature of the recording film, as shown in FIG. 10(c), will be abruptly increased at the leading edge by the wider leading pulse, but thereafter suppressed from increasing at the trailing edge by laser power irradiation with the pulse string. In consequence, the shape of the recorded mark is at good symmetry between the leading and trailing edges as shown in FIG. 10(d), thus reducing the tear-like distortion in the recorded mark.
The above-described recording method of conversion into a multiple-short-pulse string is quite effective when the linear velocity is low or the recording frequency is low. However, for high linear velocities or high frequencies of recording signals, the method has proved to involve problems newly encountered.
When the recording signal waveform is converted into a plurality of short pulses, the energy of the laser output power applied to the recording film will be reduced as compared with the direct conversion method, requiring a greater recording power level Pp. This would not matter for lower linear velocities, but when greater recording power is required with higher linear velocity, a higher laser beam output power is required to be supplied to the recording film, resulting in an increased cost of the recording system.
Also, when converting the recording input signal into a pulse string of multiple short-width pulses, it is necessary to use a clock signal having a period of some integral fraction of the pulse period of the input signal (T for the aforenoted EFM signal). Thus, when an input recording signal is high in frequency, the frequency of the clock signal becomes too high to implement a circuit design of the recording system. Moreover, laser power output involves the greater distortion in waveform when such higher frequency modulation is done.
In general use of optical discs, the relative linear velocity of the laser beam spot becomes higher at the outer circumference than that at the inner circumference of the disc surface under the condition of a constant rotating speed, i.e., constant angular velocity of the optical disc (hereinafter, abbreviated as CAV). Further, in order to increase the recording density by equalizing the length of the recording mark between the inner and outer circumferences of the disc surface, there has been proposed a method for increasing the recording frequency higher toward the outer circumference. Besides, even in the case where an optical disc is rotated at a constant linear velocity in its all regions (hereinafter, abbreviated as CLV), when a different type of optical disc is used in the same recording system, it is necessary to vary the linear velocity and recording frequency depending on the type of the disc.
Thus, the present inventors, over investigations in details in view of the foregoing points, have found that, when in overwriting a pulse-width-modulated digital signal onto an optical disc by using a single laser beam spot, it is quite effective for solving the foregoing problems to correct the recording input signal into a modulation pattern for modulating a laser power waveform into an optimum shape depending on change in linear velocity.